As is well known in the art, above ground steel reinforced concrete structures suffer from corrosion induced damage mainly as the result of carbonation or chloride contamination of the concrete. As the steel reinforcement corrodes, it produces byproducts that occupy a larger volume than the steel from which the byproducts are derived. As a result, expansion occurs in the concrete around reinforcing steel bars. This causes cracking and delamination of the concrete cover over the steel. Typical repairs involve removing this patch of corrosion damaged concrete from the reinforced concrete structures. It is good practice to expose corroding steel, at the area of damage, and to remove the concrete (e.g., break it up an remove) behind the corroding steel. The concrete profile is then restored with a compatible cementitious repair concrete or mortar, for example. The concrete then consists of the “parent” concrete (i.e., the remaining original concrete) and the new “patch” repair material.
The parent concrete, adjacent the repair area, is typically likely to suffer from some of the same chloride contamination or carbonation that caused the original corrosion damage. It is to be appreciated that steel corrosion still remains a risk in the parent concrete. Corrosion in concrete is an electrochemical process and electrochemical treatments have been used to treat this corrosion risk. Examples are described in WO 94029496, U.S. Pat. No. 6,322,691, U.S. Pat. No. 6,258,236 and U.S. Pat. No. 6,685,822.
Established electrochemical treatments generally include cathodic protection, chloride extraction and re-alkalisation. These have been classed as either permanent or temporary treatments. Permanent treatments are based on a protective effect that is only expected to last while the treatment is applied. An example of a permanent treatment is cathodic protection. The accepted performance criterion can only be achieved while the treatment is applied (BS EN 12696:2000). Chloride extraction and re-alkalisation are examples of temporary treatments (CEN/TS 14038-1:2004). Temporary treatments rely on a protective effect that persists after the treatment has ended. In practice, this means that an applicator treats the structure and thereafter hands a treated structure back to a client or customer at the end of a treatment contract.
Electrochemical treatments may also be classed as either impressed current or galvanic (sacrificial) treatments. In impressed current electrochemical treatments, the anode is connected to the positive terminal and the steel is connected to the negative terminal of a source of DC power. In galvanic electrochemical treatments, the protective current is provided by one or more sacrificial anodes that are directly connected to the steel. Sacrificial anodes are electrodes comprising metals which are less noble than steel with the main anodic reaction being the dissolution of a sacrificial metal element.
In the galvanic protection of steel in concrete, when the sacrificial anode is connected to the steel, the natural potential difference between the sacrificial anode and the steel drives a protective current. The protective current flows, as ions, from the sacrificial anode into the parent concrete and to the steel, and then returns as electrons through the steel and a conductor to the sacrificial anode. The convention of expressing the direction of current flow, as the direction of movement of the positive charge, is used in this description.
Sacrificial anodes for concrete structures may be divided into discrete or continuous anodes (U.S. Pat. No. 5,292,411). Discrete anodes are individually distinct elements that contact a concrete surface area that is substantially smaller than the surface area of the concrete covering the protected steel. The anode elements are normally connected to each other through a conductor that is not intended to be a sacrificial anode and are normally embedded within cavities in the concrete (ACI Repair Application Procedure 8—Installation of Embedded Galvanic Anodes (www.concrete.org/general/RAP-8.pdf)). Discrete sacrificial anode systems generally include an anode, a supporting electrolyte and a backfill. An activating agent, to maintain the activity of the sacrificial anode, may be included. The backfill provides space to accommodate the products of anodic dissolution and prevent disruption of the surrounding hardened concrete. Discrete sacrificial anodes have the advantage that it is relatively easy to achieve a durable attachment between the anode and the concrete structure. This is typically achieved by embedding the anode within a cavity formed within the concrete.
Galvanic protection of steel in concrete, using embedded discrete anodes, differs from sacrificial cathodic protection of steel in soil and water (BS EN 12954:2001). Anode assemblies that are embedded within concrete must be dimensionally stable as concrete is a rigid material that does not tolerate expansion of any embedded assembly. Anode activating agents are specific to concrete or need to be arranged in a way that would present no corrosion risk to the neighbouring steel (WO 94029496 or GB 2431167). Anodes are located in the concrete relatively close to the steel and embedded anodes are generally small (e.g., a discrete anode assembly diameter is typically less than 50 mm), when compared to anodes in other environments. Galvanic protection criteria, for atmospherically exposed concrete, differ from those for the cathodic protection of steel in soil or water.
One problem with the use of sacrificial anodes, in galvanic treatments, is that the power to arrest an active corrosion process on steel in concrete is limited by the voltage difference between the sacrificial anode and the steel. This problem is greatest for discrete sacrificial anode systems where, in order to protect relatively large surfaces of steel, large currents are required from relatively small anodes. A compact discrete anode will typically deliver current into an area of parent concrete, adjacent to the anode, that is one tenth to one fiftieth of the area of the steel it is expected to protect.
A number of methods have been recently proposed to increase the power of sacrificial anodes in concrete using a form of impressed current (see for example WO 05106076, U.S. Pat. No. 7,264,708 and GB 2426008). Some early teaching also exists on increasing the power of a sacrificial anode in sacrificial cathodic protection applications applied to steel in soil and saline water where different protection criteria apply (U.S. Pat. No. 4,861,449).
In WO 05106076, a sacrificial anode assembly is formed by connecting the cathode of a cell or a battery to a sacrificial anode. In one arrangement, the sacrificial anode forms the casing of a cell where the cathode of the cell is adjacent to the cell casing. An alkaline cell commonly has this property. The anode of the cell is then connected to the steel. The problem with this arrangement is that the sacrificial anode is not connected to the steel and the charge capacity of a cell is substantially smaller than the charge capacity of a similarly sized sacrificial anode. Because the anode is not connected directly to the steel, the anode cannot continue to deliver a protective current once the charge capacity of the cell has expired.
In U.S. Pat. No. 7,264,708, an automated means is provided to connect a sacrificial anode to the steel after a power supply or battery driving current from the sacrificial anode to the steel has expired. In the example in this disclosure, diodes are used to provide the sacrificial anode to steel connection. The problem with this arrangement is that power is required to achieve such a connection and this reduces the power of the protective effect. A typical diode (e.g., a silicon based diode) will use a voltage of 0.6 V to become a conductor and there is not sufficient voltage within a typical sacrificial anode system to drive a substantial current through such diodes. Another problem with this arrangement is that the power supply is located away from the anodes and is connected to the anodes with electrical cables that have to be maintained and protected from the environment and also from vandalism.
GB 2426008 (U.S. patent application Ser. No. 11/908,858) discloses a new basis for corrosion initiation and arrest in concrete that relies on an acidification-pit re-alkalisation mechanism. A temporary electrochemical treatment is used to deliver a pit re-alkalisation process from sacrificial anodes before the anodes are manually connected to the steel. The pit re-alkalisation process arrests active corrosion by restoring a high pH at the corroding sites. The pit re-alkalisation process (e.g., temporary impressed current treatment) typically lasts less than 3 weeks. The corrosion free condition is then maintained with the low level galvanic generation of hydroxide at the steel. The switch between the impressed current and galvanic treatments is achieved manually and this is facilitated by the limited duration of the temporary impressed current treatments. The power supply and the electric cables used for the temporary impressed current treatment are removed from the site. The problem with this disclosure is that the temporary impressed current treatment generally requires a skilled operator.
Another problem with discrete sacrificial anode systems is current distribution. This problem is greatest for anodes that are tied to exposed steel in cavities formed within the concrete at areas of the concrete repair. A number of solutions have been proposed to improve the current distribution from an anode tied to the steel (GB 2451725, WO 05121760 and WO 04057056, for example). However these solutions are all based on restricting the current flow to the nearest steel by increasing the resistance for current to flow to the nearest steel.
The problem to be solved by this invention is to increase the initial power, available from a sacrificial anode assembly, in order to arrest an active corrosion process while the sacrificial anode is connected to the steel in the concrete, and to improve current distribution from a sacrificial anode, connected to the steel, by directing an increased current away from the nearest steel.